I will definitely use a blower and also i am thinking of connecting it with thermal resistor to start at 40 celsius
Or maybe less than 40?

Depends upon whether that's ambient or heatsink temperature.

 

it is for heatsink temperature

 

it is for heatsink temperature

Then 40C to 50C would be a good number.

 

I decided not to use the big transformer and use this one at below pic.
It is 24V and i have the desired voltage. Not more than 32V dc





Tested again with this transformer and everything ok.

I found out that one of the transistors get very hot when i drain 4A and the other one is barely hot...
The Mj15003 connected to R3 gets very hot

Is this normal?

 

............................
I found out that one of the transistors get very hot when i drain 4A and the other one is barely hot...
The Mj15003 connected to R3 gets very hot

Is this normal?

No. It sounds like the transistor connected to R3 is hogging most of the current, perhaps due to it having a significantly higher gain then the other transistor. You may have to increase the value of R2 and R3 to better balance the current.

What are the voltages across R2 and R3 when you are drawing 4A?

 

.......I decided not to use the big transformer and use this one at below pic.......

I see that as a wise choice. If most of your intended uses for your power supply is 12-15 volts then any voltage above 18 volts is wasted creating heat and stress on your pass transistors.

 

With all linear power supplies, the voltage NOT being used, times the load current is the heat the pass transistor will dissipate. So, 4 amp times 18 volts equals 72 watts.

Considering your very helpful above reply this decision was taken

 

I found out that one of the transistors get very hot when i drain 4A and the other one is barely hot...
The Mj15003 connected to R3 gets very hot

Is this normal?

Check both transistors.
If the emitter resistors are same value than the transistors share current almost equally.

 

I am back with some updates...

Want to make it a bit modern PS and used the transistors below.
2sc 5200

Bought some new heatsink for them...



And started installing them





Will use this for the fan...


Very soon i will post testing resaults...

 

I made this significant changes with the toshiba transistors and tested the PS again but the test results were almost the same as with MJ's...
one transistor connected to R3 starts to get very hot with load connected and the other is not hot
Have some pics with the temp of eatch 2sc5200



and the other connected emiter to R2



The voltage of the one that gets very hot on emiter is 13.77V

and at the other is 14.10V



The resistors on emiters are 15W 0.22Ohm

Must i use a 0.33ohm resistor or 047ohm to the one that drains less current ?
Will this help ?

 

No. It sounds like the transistor connected to R3 is hogging most of the current, perhaps due to it having a significantly higher gain then the other transistor. You may have to increase the value of R2 and R3 to better balance the current.

What are the voltages across R2 and R3 when you are drawing 4A?

The voltage of the one that gets very hot on emiter is 13.77V

and at the other is 14.10V



The resistors on emiters are 15W 0.22Ohm

 

Check both transistors.
If the emitter resistors are same value than the transistors share current almost equally.

The transistors are changed so i suppose no need to check them... ?


Both Emitter resistors are 0.22ohm 15W
Must i unsoldier them and see what the value is with ohm meter

 

The transistors are changed so i suppose no need to check them... ?


Both Emitter resistors are 0.22ohm 15W
Must i unsoldier them and see what the value is with ohm meter

The voltage of the one that gets very hot on emiter is 13.77V

and at the other is 14.10V



The resistors on emiters are 15W 0.22Ohm

See that all the wires of the transistors are exactly the same length.
Yes, check that the emitter resistor values are as equal as possible. What I do when paralleling bipolar transistors, is to take my transistor tester with me to the shop and then I compare the actual individual gain of each (hFE). This cured a similar problem for me. I had to parallel at least 6 transistors (sometimes 8) like this.

 

Double check your wiring. It is almost like only one transistor is being used.

 

If for instance the wires from one of the two transistors differ in length and gauge from the wires of the second transistor, and if their respective hFE differs appreciably, then only one will be conducting a lot more than the other one.

 

The problem of one transistor draining all the current is resolved and as in most cases it was very simple...

Stupid me forgot to soldier the junction between the emitter and resistor



Now the temp is equall and both are running more cooler !

Transistor 1


Transistor2


And runing with lamp load...


Very happy that it is working as supposed to

 

Changed planes for this PS and will use two transformers for non heating issues.

I have purchased a 12V 10A that will be used for low voltage and have kept the 24V 5A in the case for 15 Volts to 30 Volts
Tested the 12V transformer PS under 4A load and it dosent get hot at all.


The internal of the case will be like this:



The transformer selection can be done with a simple ON-ON switch but i want to use a 10A relay that i have for the transformer to be selected automatically by the voltage change from potentiometer

Problem is that i can't figure how to switch on the relay when 15V are selected from pot.
My relay is 24 Volts and it changes state from 12V...

Simple Switch connections:


Maybe if i use a transistor as a switch will do the job to activate relay at 15V dc ?
something like this :

 

The problem of one transistor draining all the current is resolved and as in most cases it was very simple...

Stupid me forgot to soldier the junction between the emitter and resistor

View attachment 74604

Now the temp is equall and both are running more cooler !

Transistor 1
View attachment 74605

Transistor2
View attachment 74606

And runing with lamp load...
View attachment 74607

Very happy that it is working as supposed to

Congratulation, you reached it.
Most of problems are wiring, even me also happened like that, sometimes we have to using multimeter to measure the connections of wires .

 

I did have a schematic tht uses an OPAMP as a comparator to switch Tx transformer to a higher voltage when output rises.
Just dunno where I had it stored.
I believe LM393 comparator is a much simpler approach to sense the output and activate ur transistor switch to switch the second Tx

Try googling over voltage comparator. You will find plenty.

PS. If you plan to switch the Tx make sure the capacity voltage is properly rated and high capacitance to store discharge for a few ms.

 

After that i connected everything to test and the test rezualts were very disapointing...

with 4A load in about 30sec the MJ15003 gets very HOT

View attachment 74463

Very HOT

View attachment 74465

My load 50W 12V


View attachment 74466


View attachment 74467

I am very disappointed an think that this circuit is not good at all
Only the transistors get very hot.
Not the L200 either the emitter resistor.

Except the above problem with a very slight movement of the potensiometer the voltage changes very fast.
Maybe i have made a mistake with connections but i have double checked it

I managed to solve the issue of Voltage adjustment by replacing the potentiometer with a
multi turn:



Now the voltafe adjustment is Very Very smooth
I like this pot alot...